1. Field of the Invention
The present invention relates to a wobble signal detection circuit and an optical disk device. More specifically, the present invention relates to a wobble signal detection circuit for detecting a wobble signal recorded on an optical recording medium such as a CD (compact disk), a DVD (digital video disk), a DVR+R (DVD+recordable), and a DVD+rewritable), and relates to a disk device including this wobble signal detection circuit.
2. Description of the Related Art
An information recording/reproducing device (for example, an optical disk device) for recording information on an optical disk that is an optical recording medium having a spiral recording region, and reproducing information recorded on an optical disk, by using laser light emitted from an optical pickup of the information recording/reproducing device has been put to practical use.
Recently improved functions of a personal computer enabled the personal computer to process audio and visual information such as music and an image. Since an information amount of the audio and visual information is substantially large, an optical disk is considered as an important information recording medium. The price of the optical disk became cheap, and an optical disk device has prevailed as peripheral equipment of the personal computer.
Generally, a groove referred to as a track (pre-groove) is provided in advance on recording regions of a write-once optical disk such as the DVD+R and a rewritable optical disk such as the DVD+RW. Furthermore, a wobble is provided to this track in order to record various additional information as a wobble signal.
Of the additional information, ADIP (ADress In Pregroove) information is the most important. The ADIP information includes address information indicating a position on the optical disk. The address information is necessary for accurately controlling the position of the optical pickup when recording or reproducing is performed. The ADIP information further includes information that synchronizes with a rotational speed of the optical disk so that information can be accurately recorded at a predetermined position on the optical disk.
If the ADIP information cannot be accurately detected, the optical disk device cannot perform an operation that synchronizes with the rotation of the optical disk, and a recording error can occur. Particularly, in the case of the write-once optical disk, if the recording error is generated on the optical disk, this optical disk cannot be reused. For this reason, it is substantially important to accurately detect the ADIP information, i.e., the wobble signal.
The light reflected from the track of the optical disk includes the wobble signal. The reflected light, however, includes noises caused by changes of data having been recorded on the optical disk or changes of the laser light power. In the conventional manner that treats this problem, two light receiving elements that are divided from each other in a direction tangential to the tack receive the reflected light from the track. The difference between the signals (electric signals converted from light signals) output from the two light receiving elements is obtained in order to remove the noise component to extract the wobble signal. Before the shipment of the optical disk device, the attached position of the two light receiving elements is adjusted such that the reflected light from the track is received at the centers of the receiving surfaces of the two light receiving elements. However, the position at which the reflected light is received can be shifted from the centers of the receiving surfaces of the two light receiving elements due to position changes with time or age caused by temperature changes or vibrations during the operation of the optical disk device. In this case, since the noise components included in the signals output from the two light receiving elements are different from each other due to the positional changes, even if the difference between the signals output from the two light receiving elements is obtained, a certain amount of the noise component remains. Accordingly, the signal-to-noise ratio of the wobble signal decreases, and it becomes difficult to accurately detect the wobble signal.
In order to deal with this problem, according to an optical disk device disclosed in Japanese Laid-Open Patent Publication No. 8-194969, an constant amplitude AGC (automatic gain control) performs gain control so as to normalize the amplitudes of the output signals from the two light receiving elements that are divided from each other in a direction tangential to the track, and then, the wobble signal is detected based on the difference between the normalized amplitudes.
In the optical disk device, data “1” and “0” correspond to a mark (pit) region and a space region, respectively. The reflectance of the mark region is different from the reflectance of the space region. In some cases, a method of forming the mark region and the space region differs depending on a type of the optical disk device.
For example, at the time of forming the mark region on a phase change type medium such as the DVD+RW that includes specific alloy at the recording layer, the temperature of the specific alloy is raised to a first temperature by laser light, and then, is rapidly cooled by reducing the power of the laser light so that the specific alloy can be amorphous. Meanwhile, at the time of forming the space region of the phase change type medium, the temperature of the specific alloy is raised to a second temperature lower than the first temperature, and then, is gradually cooled so that the specific alloy can be crystalline. In this manner, the reflectance of the mark region becomes lower than the reflectance of the space region. In this case, as shown in FIG. 1, the intensity of the average laser light power at the time of forming the mark region is approximately equal to the intensity of the average laser light power at the time of forming the space region.
On the other hand, at the time of forming the mark region on a pigment type medium such as the DVD+R that includes organic pigment at the recording layer, the pigment is heated and melt by increasing the laser light power so that the part of the substrate contacting with the pigment can be transformed. Meanwhile, at the time of forming the space region on the pigment type medium, the laser light power is set to be small power approximately equal to the data reproducing laser light power so that the substrate cannot be transformed. In this manner, the reflectance of the mark region becomes lower than the reflectance of the space region. In the case of the pigment type medium, as shown in FIG. 1, the intensity of the laser light power at the time of forming the space region is much lower than the intensity of the laser light power at the time of forming the mark region. To be specific, as one example, the intensity of the laser light power for forming the space region is about 1.5 mW while the intensity of the laser light power for forming the mark region is about 30 mW.
According to the above-mentioned Japanese Laid-Open Patent Publication No. 8-194969, at the time of reproducing data on both the phase change type medium and the pigment type medium, it is possible to accurately detect the wobble signal. This is because the intensity of the laser light power at the time of reproducing the data is approximately constant, and the changes of the output signals from the light receiving elements are caused by only changes in the reflectance of the optical disk. In this case, the levels of the output signals from the light receiving elements are small, so that the levels of the wobble signals are also amplified by the gain adjustment performed at the constant amplitude AGC circuit.
However, in the case of recording information on the pigment type medium, the intensity of the laser light power for forming the mark region is largely different from the intensity of the laser light power for forming the space region, so that the levels of the output signals from the light receiving elements corresponding to the mark region can be 20 times larger or still larger than the levels of the output signals from the light receiving elements corresponding to the space region. That is, since the levels of the output signals from the light receiving elements corresponding to the mark region are large, the constant amplitude AGC circuit adjusts the gain such that the adjusted signal levels are in the allowable voltage range (dynamic range). Accordingly, the gain at the time of recording becomes smaller than the gain at the time of reproducing, so that the wobble signal component corresponding to the space region becomes undistinguishable from the noise component. Furthermore, at the time of forming the mark region of the pigment type recording medium, since the heat is accumulated in the recording film, the length of the mark region tends to become longer, the laser light power for forming the mark region is switched to be the lower power before the region irradiated by the laser light is actually switched from the mark region to the space region, as shown in FIG. 1. For example, when the ratio between the area of the mark regions and the area of the space regions is 1 to 1, the ratio between the time for which the laser light is emitted at the higher power and the time for which the laser light is emitted at the lower power is about 0.7 to 1.3. The time for which the signals (including the undistinguishable wobble signal components due to the lower gain) corresponding to the higher power laser light are output from the light receiving elements is considerably shorter than the time for which the signals (including the distinguishable wobble signal component due to the higher gain) corresponding to the lower power laser light are output from the light receiving elements. Accordingly, a part of the wobble signal components can be intermittently obtained by the gain adjustment performed at the constant amplitude AGC circuit because the only wobble signal corresponding to the space regions is sufficiently amplified by using the gain controlled by the constant amplitude AGC. That is, the wobble signal cannot be accurately detected.
Even during the recording on the optical disk, it is necessary to rewrite managing information recorded at a predetermined position of the recording region at a predetermined timing. At the time of rewriting the managing information, the recording is stopped momentarily, the managing information is read and altered, and the altered managing information is then recorded at the predetermined position of the recording region. In other words, even during the recording, the reproducing is performed at the predetermined timing. However, when a response speed of the constant amplitude AGC circuit is taken into account, a certain length of time is required at the time of switching between the recording and the reproducing so that the gain can become a predetermined value. Accordingly, during the transitional period between the gain values, the accurate wobble signal cannot be obtained.